Monitoring induction motors for energy savings

Your motors can help you find substantial energy savings, if you can hear what they’re trying to tell you. The right monitoring equipment will help you get the message.

Jim Plourde, Schneider Electric

11/01/2011

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With limited natural resources and the ever-increasing global demand for energy, it only stands to reason that energy costs will continue to increase. As energy costs rise around the world, the incentives for facilities to operate their equipment more efficiently will compound over time. There are mandatory means (e.g., regulations) that authorities use to enforce conservation, and compensatory means (e.g., special rate tariffs) that reward users for using less energy. In either case, reducing energy consumption will decrease bottom-line costs to the user.

To reduce energy costs effectively, industrial facilities should begin by evaluating their No. 1 energy consuming culprit: motors. Many studies have shown that motors in industrial facilities consume by far the largest percentage of energy of any electrical device. According to the U.S. Department of Energy, motor-driven equipment accounts for 64% of the electricity consumed in the U.S. industrial sector—consuming approximately 290 billion kilowatt hours (kWh) per year. The most common type of industrial motor in use today is the three-phase (polyphase) induction motor, over 90% of which are squirrel cage induction motors.

Using three-phase over single-phase power systems offers an exponential increase in energy transmission efficiency. The power transmitted in polyphase systems is calculated as the voltage multiplied by the current in each conductor, times the square root of three (approximately 1.73), whereas the power transmitted by a single-phase system is simply the voltage multiplied by the current. As a result, three-phase systems transmit 73% more power while using only 50% more conductor. Given their prevalence throughout industrial and commercial sectors, polyphase induction motors offer a great potential savings opportunity in both energy and operational costs during the motor’s useful life.

An adequate assessment of the impact that induction motors can have on your energy bill requires a detailed knowledge of the motor’s many operational and electrical parameters. Permanently installed monitoring devices are the most effective tool in the arsenal to reduce energy consumption, especially in motors. Knowing which parameters to monitor and evaluate will help maximize energy savings.

Monitoring motors

Each motor within a facility operates with some level of distinctiveness from other motors. This distinctiveness may be due to a mixture of factors including: nameplate ratings, voltages, load/application, duty cycle, environment, adjacent loads, impedances, and age.

The more knowledge that can be accumulated about a motor, its associated load, and how it operates, the easier it is to reduce energy costs associated with that motor. Power monitoring devices provide real-time and historical data needed to operate motors at peak efficiency and maximize their useful life. Such devices can be installed either temporarily or permanently, depending primarily on the nature of the motor load, accessibility of the motor and conductors, and cost.

Temporary monitoring is best suited for studying noncritical motors and those that do not use enough energy to warrant dedicated monitoring equipment. Temporary monitoring may also be beneficial for power system planning and event troubleshooting.

While permanent monitoring of every motor may be impractical, permanent monitoring of mission-critical motors enables operators to identify and avoid problems in advance of motor failure and is advised for motors that use significant amounts of energy or are located in remote or otherwise inaccessible locations.

Permanently installed monitoring systems are particularly useful because they are able to capture a great deal of information, both real-time and historical, over the motor’s life. By monitoring the voltage, current, and temperature, today’s monitoring devices can provide data on many aspects of an induction motor including:

Quality of the motor’s terminal voltage

Energy usage

Loading concerns

Excessive cycling

Starting characteristics, and

Environmental considerations and maintenance.

A fundamental issue that can affect a motor’s energy usage is its suitability for the intended application. Motors are designed to operate most efficiently at their nameplate rating. Selecting the wrong motor for a particular application or operating the motor outside its recommended parameters will decrease the motor’s performance, introducing additional losses into the electrical system. Monitoring systems are able to identify many symptoms that result in reduced motor performance including deviations from various nameplate parameters. For example, Figure 1 illustrates several consequences that occur when the voltage deviates from the motor’s nameplate voltage rating.

Where to find savings

There is a wealth of information about a motor’s well-being buried in the characteristics of the electrical signals at the motor’s terminals. With the motor’s nameplate data and these electrical characteristics, it is possible to quantify many energy-saving opportunities for a given motor. The fundamental electrical characteristics include the voltage, current, and frequency data for each phase. By collecting data on these fundamental characteristics, monitoring devices can provide additional information needed to maximize energy savings including:

Power factor

Voltage variations

Voltage imbalance

Motor load (based on current)

Harmonic distortion, and

Frequency deviations.

Monitoring systems also have the ability to measure and record temperatures, number of starts, running time, and even vibration through the use of I/O modules providing guidance for operational guidelines, preventive maintenance, and predictive failure analysis.

Power factor improvement

The first and most obvious opportunity for motor energy savings is power factor correction. The power factor of an ac electrical system is the ratio of the “real” power going to the load to the “apparent” power in the circuit. Loads with a low power factor must draw more current than a load with a high power factor for the same amount of useful energy transfer. Most monitoring systems provide a wide range of data directly or indirectly associated with understanding power factor including:

Displacement power factor (total and per phase)

True power factor (total and per phase)

Distortion power factor (total and per phase)

Min/max power factor

Reactive power and energy

Real power and energy, and

Apparent power and energy.

Power factor can lead to energy savings by leveraging polyphase induction motors that use current composed of both resistive and inductive components shown in Figure 2. The resistive component includes the load current and the loss current; the inductive component includes the magnetizing current and the leakage reactance. It is possible to cancel out the inductive current component by supplying a counter current using a capacitor. The addition of a capacitor does not affect the magnetizing current or the leakage reactance of the motor, but it offsets the inductive component at the point where the capacitor is installed. As more capacitance is added, the power factor angle, θ, becomes smaller until a unity power factor is achieved (θ = 0). At unity power factor, the electrical system is at its optimum performance for maximum power transfer. Please note that placing excessive capacitance on the circuit will result in a leading power factor (θ is negative in this case), which can lead to serious complications.

First example

A three-phase induction motor uses 200 A at a power factor of 0.78 (θ old = 38.73°).

To ensure these values are correct,

The reactive (inductive) component can be reduced by adding a capacitive load (generally a capacitor bank) near the motor. The capacitive load is also expressed as reactive in nature, but it uses the current 180 degrees out of phase from the inductive load; thus, a canceling effect occurs, shown in Figure 3. A properly sized capacitor bank could bring the power factor from 0.78 (θ old = 38.73 degrees) to 0.95 (θ new = 18.19 degrees), resulting in a reduction in current of approximately 18% based solely on the power factor improvement. Each kVArh of reactive energy passing through your electrical system produces superfluous line losses and higher energy bills. Permanently installed monitoring devices can quantify these losses and offer additional savings opportunities within the facility.

A word of caution: Most industrial systems use motors with adjacent loads that are complex (e.g., nonlinear loads such as adjustable speed drives). These complex load-types may react negatively to the addition of standard power factor correction capacitors due to the capacitors’ interaction with other frequencies produced by the complex loads. It is recommended that users take a close look at information available regarding the interaction between complex-load types and power factor correction capacitors (also see displacement power factor versus true power factor).

Voltage imbalance

Voltage imbalance (including single phasing) is both a leading cause of motor failures and a major contributor to energy losses in motors. Voltage imbalance in fully loaded polyphase induction motors produces a disproportionately higher current imbalance leading to increased motor temperature rise. This increased heating shortens motor life by breaking down its insulation. Monitoring systems quantify voltage imbalance for power quality purposes but may also be used to provide information on the losses due to voltage imbalance at the terminals of three-phase induction motors. Figure 4 illustrates the effect of voltage imbalance on a motor’s efficiency.

Second example

A 200 hp three-phase induction motor operates 4,500 hours each year at an average load of 80%. The motor’s efficiency (η) is 93% at an 80% load, assuming a negligible voltage imbalance. However, after reviewing data from the monitoring system it is discovered that the average voltage imbalance to the motor over the course of year has been 3%. The facility’s average energy cost is $0.13/kWh and the average demand charge is $16/kW.

The reduction in efficiency, shown in Figure 4, is roughly 3.5% giving the new efficiency (η new) as 89.5% (93% – 3.5%). The losses due to the voltage imbalance are determined as follows:

To determine the total cost due to the voltage imbalance each year,

The cost of energy losses is substantial in this case and is further multiplied by additional motors exposed to the voltage imbalance within the facility. Moreover, other voltage quality issues also adversely affect the efficiency of induction motors. Operating a motor at 90% of its rated nominal voltage will result in roughly a 2.5% decrease in efficiency (see Figure 1).

Harmonic distortion at the motor’s terminals produces additional currents including counter-rotational (negative sequence) currents that reduce a motor’s efficiency. Even variations in the system frequency result in energy losses for motors. All of these factors, and many more not mentioned in this article, contribute to losses and represent an untapped means of reducing operational expenditures.

Historical and real-time data provided by monitoring systems is the key to locating motors that are operating uneconomically, but these systems allow you to determine the root cause of problems more easily. Remedies can also be assessed on an as-needed basis by the monitoring system, and modified to insure they are effective. Concurrently, the return on investment (ROI) for a given solution can be easily established.

Acting on the knowledge

Permanently installed monitoring systems collect vast amounts of data that can be scrutinized for motor savings. In general, actions taken to improve the motor’s efficiency will also increase the operating life of the motor. The payback period for improvements to the electrical system, including the initial costs of installing a monitoring system, can be relatively short—especially if multiple motors are impacted. As steps are taken to operate motors as closely as possible to their optimal parameters, the effects will be decreased capital expenses, less process downtime, lower stress on the supporting infrastructure, and, of course, reduced operating expenses, including energy bills.

In the end, true ROI from monitoring systems is achieved by acting on the information collected. Implementing the monitoring capabilities is the first step. Through active energy management—the concept of automating, metering, monitoring, and continuously commissioning—organizations can ultimately reduce their current energy use by 15% to 30% right now.

Jim Plourde is national business development manager of energy solutions for Schneider Electric.